Arthropods serve as vectors for several emerging and reemerging infectious disease causing agents including Rickettsia felis. Since first being described in colonized cat fleas, R. felis has also emerged as a human pathogen. To understand the potential risk of this pathogen, we must first understand the biology of the organism. Our long-term objective is to delineate the role of the flea vector in the maintenance and transmission of R. felis in nature. Defining the mechanisms of transmission within populations of fleas is critical for our understanding of the epidemiology of flea-borne spotted fever and widely impacts our approach to control measures. The proposed studies will assess the role of invertebrate and vertebrate hosts in transmission of R. felis via the specific aims: (1) delineate the mechanisms by which R. felis spreads through individual cat fleas, Ctenocephalides felis, and throughout flea populations. Parameters of R. felis infection and transmission in fleas;specifically, a) horizontal transmission (contribution of vertebrate hosts and concurrent feeding);b) vertical transmission (dynamics of replication and dissemination through adult tissues and immature stages of life cycle);c) flea fitness (fecundity and eclosion rates);and d) sex ratio distortion (comparative analysis of male:female ratios) will be measured in a controlled laboratory R. felis/C. felis model;and (2) examine the ecology of flea-borne rickettsioses by assessing the influence of the vertebrate host bloodmeal and flea species on vector competence. Studies proposed will assess the vector competence of cat fleas for R. felis in the presence of alternate bloodmeal sources, as well as the competence of flea species other than cat fleas to acquire and transmit R. felis. Rickettsioses are diseases caused by members of the typhus group and spotted fever group of Rickettsia. Pathogenic Rickettsia are typically transmitted to humans by arthropods (insects and ticks). This project seeks to examine the interplay between Rickettsia felis and the vector flea, to better understand the ecology of flea-borne spotted fever.